de Queiroz response to Wagner
In a posting dated 6/1/00, [Jonathan Wagner] wrote:
" Indeed, there is (as far as I can see), a much more grave
potential problem with the first definitional format. Consider the following
species phylogeny (using the same species discussed previously, where each
letter is a species):
A D'
D
D E
D F'
F F'
FF' B
F GB
C G
CG
C
(read as: "species A is sister to species D', both descended from
species D, species D is descended from species F, as is species F'. both
species F and species G are descendants of species G, and species B is a
descendant of species G.)
Clade X is defined as "species A and all species sharing a more
recent common ancestral species with A than with B." Species A, D', D, E,
and F' all share a more recent common ancestral species with species A than
with species B. However, species F does not. The most recent ancestral
species of species F is species C, which is also ancestral to species B.
Therefore species F is not included in the taxon. The result is a
polyphyletic group."
>Ah yes, now I remember this posting, though I was too busy to respond at
the time. The problem here stems from reference to a common ancestral
species (for more on this problem see de Queiroz and Donoghue, 1988,
Cladistics 6:61-75, especially Fig. 2), so all that has to be done is to
delete that term from the definition (i.e., "A and everything sharing a more
recent common ancestor with A than with B"). Note that this is how the
definition is stated (i.e., without reference to an ancestral species) in
the example given in the PhyloCode (Note 9.4.1). No change is necessary.
>[part of original KdQ message] nor do I see any ambiguity with the seond
phrasing.
Please see the discussion between Dr. Wolsan and I, in which part
of his point was that referencing a "CLADE" as opposed to "an ancestor and
all of its descendants" or "and ancestral species and all of its
descendants" allows one to separately define what you consider a clade to be
(e.g., a species and all of its descendants vs. a breeding pair and all of
its descendants). The result is that the group identified by the definition
then changes based on different interpretations of the word clade. This is
an ambiguity which I do not find helpful, since I think you will agree that
a breeding pair and all of its descendant organisms is a different group
than a species and all of its descendant species. I want my Lepidoptera to
be the same as everyone else's lepidoptera... isn't that the point, after
all?
Personally, I prefer to have it spelled out: "an ancestral species and all
of its descendants." No ifs, ands, or buts that I can see in that.
The problem is that other people prefer not to spell it out that way, and
some of those people were contributors to the PhyloCode effort. I don't
really see this as a big problem in that most people use clade for an
ancestral species (as opposed to a breeding pair) and its descendants.
Indeed, I have more or less formalized this distinction in my 1999 paper (in
Species: New Interdisciplinary Essays: see Note 3 regarding the distinction
between clades, clans, and clones). Moreover, the term "clade" is defined
in the glossary of the PhyloCode, though not in a way that entirely solves
the problem. The simplest solution would be to modify that definition,
replacing "ancestor" with "ancestral species"--a wording that I believe I
advocated but on which I compromised to accommodate the views of the
"anti-species" contributors.
Kevin de Queiroz
30 July 2000